Surface Modification of a Nickel-Free Austenitic Stainless Steel by Low-Temperature Nitriding

Francesca Borgioli,Tiberio Bacci,Emanuele Galvanetto

doi:10.3390/met11111845

Abstract

Low-temperature nitriding allows to improve surface hardening of austenitic stainless steels, maintaining or even increasing their corrosion resistance. The treatment conditions to be used in order to avoid the precipitation of large amounts of nitrides are strictly related to alloy composition. When nickel is substituted by manganese as an austenite forming element, the production of nitride-free modified surface layers becomes a challenge, since manganese is a nitride forming element while nickel is not. In this study, the effects of nitriding conditions on the characteristics of the modified surface layers obtained on an austenitic stainless steel having a high manganese content and a negligible nickel one, a so-called nickel-free austenitic stainless steel, were investigated. Microstructure, phase composition, surface microhardness, and corrosion behavior in 5% NaCl were evaluated. The obtained results suggest that the precipitation of a large volume fraction of nitrides can be avoided using treatment temperatures lower than those usually employed for nickel-containing austenitic stainless steels. Nitriding at 360 and 380 °C for duration up to 5 h allows to produce modified surface layers, consisting mainly of the so-called expanded austenite or γN, which increase surface hardness in comparison with the untreated steel. Using selected conditions, corrosion resistance can also be significantly improved.

Highlights

The face-centered cubic (f.c.c.) structure of austenite is maintained in austenitic stainless steels owing to the use of austenite forming elements
The examination of the cross-section of the modified surface layers produced on lowtemperature nitrided austenitic stainless steels, as delineated by chemical etching, showed that they consisted of an outer layer, in which expanded austenite, γN, was present, and an inner layer, consisting of a solid solution of interstitial atoms in the slightly expanded austenite lattice, γ(N,C), as previously reported [15,26]
Both these layers were a modification of the austenite matrix and their characteristics depended on the nitriding conditions

Summary

Introduction

The face-centered cubic (f.c.c.) structure of austenite is maintained in austenitic stainless steels owing to the use of austenite forming elements Among these alloy elements, nickel is widely used, since it allows to improve ductility and toughness, and it has a positive effect on corrosion resistance, promoting repassivation [1]. In order to improve biocompatibility, austenitic stainless steels with a negligible nickel content, usually known as nickel-free (Ni-free) stainless steels, have been developed [4,5,6,7,8] In these alloys, in which nickel content is lower than 0.3 wt.%, the austenite forming elements are usually manganese (9.5–24 wt.%) and nitrogen (0.45–1.1 wt.%), and molybdenum is added for increasing localized corrosion resistance [4,5,8]

Methods

Results

Discussion

Conclusion